The invention relates to a process and a device for delivering a monochromatic output light beam having a narrow spectrum linewidth; it is particularly--but not exclusively--suitable for use in photochemistry.
It is well known that intense light beams having a spectrum linewidth as narrow as possible is of great interest in numerous photochemistry fields, for instance for isotopic separation or purification operations.
There exist techniques for reducing the spectrum line-width of a beam which use passive filtering apparatus (prism monochromators, diffraction grating monochromators, interferential filters, Fabry-Perot etalons and the like). Any reduction in the spectrum linewidth is obtained at the cost of reduction of the light intensity of the beam in higher proportion. Consequently, any passive filtering operation results in a reduction in the monochromatic intensity defined as the ratio of the light intensity to the spectrum linewidth.
It is an object of the invention to provide a process and a device delivering an intense light beam of very small spectrum linewidth, the term "light" being construed as designating the whole zone of the spectrum covering the infrared, the visible and the ultraviolet (i.e. approximately from 100 nm to 100 .mu.m).
According to an aspect of the invention, there is provided a process for generation of a light beam with a narrow spectrum linewidth, comprising, superimposing an amplifying medium exhibiting a large stimulated light emission spectrum linewidth at a first frequency and a pump light beam at a second frequency, higher than said first frequency, the first frequency being contained in the stimulated emission spectrum linewidth of the medium induced by the second frequency.
Two results are attained by the process:
the intensity of the light radiation beam at the first frequency may be considerably increased without substantial degradation of the spectrum linewidth, whereby there is an increase of the monochromatic intensity;
an appreciable fraction of the energy of the beam at the second frequency, whose spectral width may be large, is transferred to the beam at the first frequency, of narrow spectral width; in other words, spectral compression is achieved with a relatively high transfer efficiency.
That process is quite different from those which use stimulated light scattering for frequency shift. The latter processes use amplification of the inherent noise (typically Raman noise) of the system so that the spectral characteristics of the radiation obtained depend on the spectrum linewidth .DELTA..sigma..sub.T of the transition in the medium and on the spectrum linewidth of the pump wave .DELTA..sigma..sub.p. On the other hand, according to the invention, a very monochromatic signal is injected, whose width .DELTA..sigma..sub.S may be very much less than .DELTA..sigma..sub.P and whose frequency is within the spectrum region where the pump wave induces a gain.
In a particular embodiment an amplifying medium presenting Raman scattering is used. The second beam forms in this case the pump beam. The first one having a much smaller spectrum linewidth than that of the transition is at a frequency equal to or close to that of the Stokes spectrum line which would be induced by the pump beam if the gain were sufficient. The two beams may be provided by pulsed lasers.
Instead of using Raman effect stimulated light scattering, other properties may be used and particularly Rayleigh scattering (with frequency change) or Brillouin scattering. Implementation of the invention by Raman effect entails refraining to reach the threshold of the Brillouin effect, since there would otherwise appear competition between the two effects.
Although lasers are radiation sources particularly suitable for implementing the invention, other sources of coherent monochromatic radiation in the useful spectrum may be used, e.g. parametric oscillator systems.
The invention also relates to a device for implementing the above-defined process. According to another aspect, there is provided a device for generation of a monochromatic light output beam having a narrow spectrum line, comprising: an enclosure for receiving a medium having a wide induced amplification spectrum line; a first pulsed light source for energy extraction selected to inject a light beam with a narrow spectrum linewidth at a first frequency into said medium; a second pulsed source for pumping action which provides in operation a light beam at a second frequency, different from the first, said first frequency being selected to be within the amplification spectrum line of the medium when subjected to the action of the pump beam; and optical means for rendering the two beams colinear in the enclosure.